In 1948, Sylvia Watson, who has died aged 91, was appointed children's officer for the new children's department in Hertfordshire. She was to build it into one of Britain's leading childcare services, reading every report written by her officers - and adding, as I soon discovered, as a young officer in the 1960s, criticisms and praise in red ink.

Sylvia knew about every child in the care of the council. Her commitment to deprived children was such that she stayed with that department from its beginning until its end, when she became, in the early 1970s, Cambridgeshire's first director of social services.

Her first task at Hertfordshire in the late 1940s was to humanise the residential homes inherited from the public assistance department. One had 100 children in dormitories with barely a space between the beds. She reduced the size of the homes and drew in staff who treated children as individuals.

Next she cut the numbers in institutions by promoting foster homes. Her experience as a wartime evacuation officer proved invaluable as she detailed the kind of people she wanted as foster carers.

But by the early 1960s the Association of Children's Officers - of which she was an enthusiastic member and president in 1967-68 - had recognised that many children need not be in care if their parents were helped at an early stage. So the ACO campaigned for local authorities to get the powers to spend money on prevention. Those powers came in 1963.

Sylvia encouraged her staff to undertake preventive work, but in 1973 she argued: "If our work is to be really preventive and promotional we must aim at remedying defects in the social structures as well as helping individuals." In short, she wanted the government to tackle poverty.

The Hertfordshire children's department became noted for the high percentage of children in foster homes. Not only was Sylvia a competent administrator but she also attracted qualified staff. She seemed to know them all and, when greeting those with families, would ask after their children by name. Staff responded by giving her their loyalty.

Born in Newmarket, she was educated at a Cambridge boarding school and then took a degree and trained as a teacher at Cambridge University in the early 1930s. Her teaching career was overtaken by the second world war.

In the early 1970s, children's departments were merged into much larger social service departments and thus did Sylvia move to the Cambridgeshire directorship. Again, she proved an inspiring leader, although in later life she expressed doubts about the reorganisation and told me that the personal emphasis on children had been lost.

She retired in 1974 and then had time to pursue her interest in the Save the Children Fund. She became its vice-president and served the Fund until old age really made her retire.

In her 80s, she lived in a flat in her beloved Cambridge and maintained contact with relatives, friends and some former children who had been in the care of Hertfordshire. Having not seen her for years, I visited her at her flat. On looking at me, she said: "Very faded at the edges, but it is you." Her twinkling eyes remained alert until the end.